In sliding vane positive displacement pumps, such pumps are used in a number of different industrial and commercial processes to force fluid movement from a first location to a second location. Generally, such a pump includes a hollow housing or casing shaped to define a pump chamber. Typically, the pump chamber has an eccentric, non-circular cross-sectional profile, preferably defined by a liner that is stationarily supported in the casing. The pump chamber is supplied with process fluid through an inlet and discharges the process fluid from an outlet at an increased discharge pressure.
In prior art pumps of this type, the opposite ends of the pump chamber are open but closed off by disc-like, first and second head plates bolted to the opposite sides of the casing. The first and second head plates sandwich the liner therebetween so as to prevent movement during shaft rotation. The shaft extends through the casing and is driven by a motor or other motive means wherein the shaft drives a rotor located within the pump chamber.
To effect pumping, the rotor may include vane slots, which are spaced circumferentially from each other and open radially outwardly. The vane slots also open axially through the opposite rotor faces toward the opposing faces of the head plates. Vanes project outwardly from the slots and are movable radially into and out of the slots so as to closely follow the inner profile of the liner. As the shaft and rotor turn, the volume of the space in the chamber between circumferentially adjacent vanes and the radially opposed surfaces of the rotor and liner (each space referred to as a fluid cavity), cyclically increases and decreases due to the eccentric profile defined by the liner.
In more detail, the shaft extends through shaft holes which are formed in the center of the head plates. A small radial gap is defined between the inside diameter of the shaft holes and the opposing outside diameter of the shaft surface, and while some process fluid might leak axially out of the pump chamber along the radial gaps, mechanical seals are provided on the opposite shaft ends to prevent leakage of such fluid out of the pump.
Each mechanical seal includes a rotating sealing ring mounted on the shaft so as to rotate therewith, and at least one stationary sealing ring, which is stationarily supported on a seal housing in opposing relation to the rotating sealing ring. One of the opposed sealing rings is axially movable so that opposing sealing faces are biased axially towards each other in sealing engagement to define a sealing region extending radially across the opposed sealing faces. The opposed sealing rings may be provided in various combinations of single or dual seals. Dual mechanical seals may be configured in one type, with axially spaced sealing rings, or in a second type, with radially spaced sealing rings wherein one or two sealing rings face two concentric, radially spaced sealing rings.
Generally in known pumps, a limited amount of process fluid may flow out of the pump chamber along the radial gaps between the shaft and head plates but such axial flow is blocked by the mechanical seals which are located axially adjacent to but spaced from the radial gaps. The mechanical seals prevent fluid from leaking along the shaft to ambient environment on the exterior of the pump.
In known configurations of this type, the operation of the pump is suitable and the mechanical seals are effective to prevent leakage. However, sliding vane pumps of this construction also exhibit fluid slip from discharge to inlet chambers within the pump chamber which reduces pump efficiency. More particularly, the head plates are located at the opposite ends of the rotor and respectively face axially toward the opposing rotor faces. Due to the relative rotation therebetween, a small axial clearance or end clearance is required between the rotor end faces and axially opposed head faces to avoid undesirable contact therebetween during shaft rotation.
Due to this end clearance, disadvantages are present. On the one hand, the opposed end faces of the rotor and head plates and the end clearances therebetween generate dynamic sealing due to the relative movement therebetween which is desirable. However, these end clearances still define fluid paths that extend face-wise across the rotor end faces and opposed head faces that allow pressurized fluid to slip from the outlet side to the inlet side of the rotor. This slip thereby reduces the overall hydraulic efficiency of the pump, since such fluid is not discharged through the outlet but instead returns to the inlet side and is then displaced again by the rotor and vanes back towards the outlet. This loss is conventionally known as slip. This slip can occur across the radial width of the rotor as defined radially from the outer shaft diameter to the outer rotor diameter.
In another aspect, the mechanical seals are located outwardly of the head plates which can increase the overall axial length of the pump. The shaft bearings in turn can be located axially outboard of the mechanical seals which also adds to the axial length of the equipment.
It is desirable to provide an improved pump and mechanical seal design which overcomes disadvantages with known sliding vane pumps and other applicable pumps.